Intermodulation is one of the phenomena that become problems in a broad frequency band for apparatuses, including an electronic apparatus operating in a low-frequency audio band and a wireless communication apparatus. Generally, intermodulation is generated as a distortion signal of a new frequency component when a low frequency electronic apparatus, a wireless communication apparatus or a semiconductor device receives or outputs a plurality of signals of different, but similar frequencies. In a low frequency band, for example, an analysis is performed based on contact, nonlinear distortion (see non-patent document 1). Further, because of requests for an increase in frequency and efficiency, and for a decrease in distortion for a wireless communication apparatus, a reduction in intermodulation has also been studied for a high frequency band. These intermodulation problems have been regarded as matters related to the nonlinearity of active components, such as an amplifier and a semiconductor device employed in the apparatus.
Relevant to the above described intermodulation, a new problem is focused on: passive intermodulation. In this specification, hereinafter, passive intermodulation is simply called PIM. Unlike the conventional intermodulation problem that occurs in active components, PIM occurs in passive circuits and passive components. It is known that PIM usually occurs at a metal contact point for different types, or for a like type, of metals due to a potential difference of the metals and a nonlinear, lumped resistance.
FIGS. 12A and 12B are diagrams for explaining the concept of PIM. PIM is a distortion signal that is generated, as is conventional intermodulation, at a time whereat two signals with similar frequencies are transmitted to an object to be measured. In FIG. 12A are shown PIM signals (f3, f5, f7, . . . ) that are generated when a test signal having a frequency f1 and a test signal having a frequency f2 are input to an object to be tested. According to this relationship, the odd-number order PIM signal appears around the two test signals, and in accordance with an increase in the number order, distortion is located in a frequency distant from the test signals. Further, in a case wherein the frequencies f1 and f2 are distant from each other, an even-number order PIM signal would become the cause of a poor reception. Generally, the PIM generation level is lower as the order number becomes greater. For simplification, only PIMs on the higher frequency side from the test signals are shown in FIG. 12A. Even in a case wherein test signals having a power level of +43 dBm are input, the generation level for PIM signals is very low, i.e., about −50 to −150 dBm. This level is very low, compared with intermodulation distortion that is a problem for an active component. However, a wireless communication apparatus that handles low level signals can not ignore this PIM.
FIG. 12B is a diagram for explaining why a PIM constitutes a communication outage. FIG. 12B shows a frequency relationship between two separated frequency bands, i.e., a transmission frequency band (Tx) and a reception frequency band (Rx), for a wireless communication system of a frequency division duplex method. For example, in a case for the base station of a mobile phone system, two signals of frequencies f1 and f2 are simultaneously transmitted in the transmission band Tx of the base station. At this time, a PIM signal of a frequency f3 is generated, for example, by a passive component, such as a transmission filter, provided in the base station transmitter and an antenna. This PIM signal falls, via a transmission/reception antenna, into the reception band Rx of the base station. In such a case, the PIM signal becomes a disturbance factor for a signal received at the same frequency f3, and a communication outage occurs.
Recently, PIMs generated in passive component materials become conspicuous, especially due to a high-frequency and high-power operation, a multicarrier technology and a broad band modulation in wireless communications. Formerly, it was considered that PIMs occurred as a result of metal potential differences and nonlinear resistance at contact point portions of passive components. However, a new focus has been drawn to the fact that PIMs occur in states wherein nonlinear lumped resistance or different types of metals are not present, e.g., a PIM that occurs in a metallic material such as is used for a printed board circuit.
FIG. 13A is a diagram for explaining a PIM that occurs at a contact point portion. An antenna 124 portion of a wireless equipment (e.g., a mobile phone system), which employs a frequency division duplex system, is employed as an example. An antenna duplexer (hereinafter referred to as a DUP) 120, which separates a transmission signal from a reception signal, is employed to share as both a transmission antenna and a reception antenna. The DUP 120 accommodates a transmission terminal 121, to which a transmission circuit is connected, a reception terminal 122, which is connected to a reception circuit, and an antenna terminal 123, which is connected to an antenna 124. When two transmission signals having different frequencies are transmitted by the transmission circuit to the DUP 120, a PIM signal 125a and a PIM signal 125b are generated at the connector contact points of the transmission terminal 121 and the antenna terminal 123.
FIG. 13B is a diagram for explaining a PIM generated in a material of a passive component. A PIM is still observed in a case wherein a plan circuit, such as a microstrip line (hereinafter referred to as an MSL), is employed for connections made to the DUP 120, instead of connectors being used that require contact points. It is assumed that the PIM is generated in a plan circuit 126a, such as an MSL and a phase shifter that are provided for the antenna 124, or a plan circuit, such as an MSL 126b that is mounted on the printed circuit board of the wireless equipment. It is also assumed that the PIM is generated in a conductive material and a dielectric material that constitute the MSL. Non-patent document 2, submitted by the present inventors, also discloses information relative to the evaluation results, obtained by a PIM measurement performed using the MSL, for the PIM that, itself, is generated in a metallic material.
FIG. 14 is a diagram illustrating a conventional PIM measurement system for a high frequency band. The arrangement of the PIM measurement system described, for example, in non-patent document 2 can be roughly divided into a section for generating a test signal, and a section for separating/detecting a PIM and an object to be tested. Test signals, for different frequencies f1 and f2, are generated by signal generation units 101 and 102. The electric power levels of the test signals must be sufficient for the generation of a PIM in a DUT. As an example, there is a case wherein a signal generator is employed that can output signals, each of which has a power of +43 dBm (20 W) or higher. The signal generation units 101 and 102 generally include a signal generator (SG) and an A class power amplifier.
The test signals transmitted by the two signal generation units 101 and 102 are combined by a coupler 103, and the obtained signal is transmitted to the transmission terminal of a DUP 107. The test signals for two frequencies, output by the antenna terminal of the DUP 107, are transmitted to one end of a sample to be measured (hereinafter referred to as a device under test (DUT)) 104. The other end of the DUT 104 is connected to a terminator 105. The test signals are terminated by the terminator 105. The terminator 105 is employed so that the impedance of the entire PIM measurement system matches a characteristic impedance of 50Ω, for absorbing a test signal, and the performance of the measurement is stabilized without generating an unwanted reflection wave around the DUT 104. According to this conventional PIM measurement method, fundamentally, the impedance of the entire measurement system is to be controlled at 50Ω, and the test signal for a high power level is to be passed through the DUT 104 and be absorbed by the terminator 105.
The PIM signal generated in the sample DUT 104 is transmitted to the antenna terminal of the DUP 107. The PIM signal is limited in the reception frequency band by the filter in the DUP 107, and the PIM signal is passed through the reception terminal and is measured as a PIM signal having the frequency f3, by a PIM measurement unit 106. The PIM measurement unit 106 includes, for example, a low noise amplifier and a spectrum analyzer. As is well known by one having ordinary skill in the art, the DUP 107 has a function for separating a transmission signal and a reception signal, and the operation thereof will not be given.
The above described conventional PIM measurement method, however, has the following problems, and is not a satisfactory method. The first problem is that the physical structure of a DUT is limited. For example, for a DUT that is disclosed in non-patent document 2 and that is employed for the PIM measurement using the MSL, a plane circuit that includes a metallic electrode material and a dielectric substrate material, for which the measurement is to be performed, must be prepared.
FIG. 15 is a diagram illustrating the structure of a measurement sample employed in a case wherein a PIM for an MSL is to be measured. As a measurement sample DUT 104, an MSL 113, comprising a copper foil conductor having a width W and a thickness t, is formed on the upper face of a dielectric substrate 112 having a width Ws, a length Ls, a thickness h and a dielectric constant ∈r. The opposite face of the MSL 113 is a ground plane made of the same copper foil as the MSL. Further, the two ends of the MSL 113 are connected to semi-rigid cables 110 and 111 that connect the DUT 104 to the DUP and the terminator. As previously described for the conventional PIM measurement method, the DUT 104 must be prepared to obtain an impedance match for a characteristic impedance of 50 Ω.
In a case wherein a transmission line is to be formed using the MSL, generally, the characteristic impedance of the line is determined based on the thickness h, the dielectric constant ∈r, for the dielectric board 112, and the width W and the conductor thickness t of the line conductor 113. In a case wherein one parameter is changed, the other parameter must also be changed in order to maintain a constant value for the characteristic impedance. Therefore, in a case involving an evaluation of the relationship of the generation of the PIM to the individual transmission line parameters described above, it is difficult for only a single parameter to be independently controlled.
A second problem is a PIM that occurs at a terminator, which is required to absorb the electric power of a test signal passed through the DUT. In the conventional PIM measurement system shown in FIG. 14, a PIM signal 108b is also generated by the terminator 105. The PIM measurement unit 106 then measures the resultant signal where the PIM signal 108a, which results from the sample in the DUT 104, and the PIM signal 108b, which results from the terminator 105, interfere with each other. At this time, when the level of the PIM generated at the terminator 105 is high, the measurement dynamic range for the PIM generated at the DUT is limited. The actual PIM generation level for the terminator is about −70 dBm, for example, for a common terminator that employs a resistor. Even for a terminator that includes a very long transmission line (e.g., a semi-rigid line of about 100 m) employing an appropriate transmission line loss with an open end, the PIM level is about −120 dBm. Therefore, when a special terminator is not prepared, PIM measurement at a low level is difficult, and it is hard for obtaining a wide dynamic range for a measurement system. In addition, since a connector contact point is included for the connection of the DUT 104 and the terminator 105, a PIM generated at the connector contact point is an element that makes the measurement uncertain. Therefore, considerable attention must be paid to the uncertain element in the measurement system. As described above, for a conventional PIM measurement system that assumes impedance matching condition, it is difficult for a large dynamic range to be stably obtained to perform a measurement.
As a problem that derives from the limitation on the structure of the above measurement sample, influence resulting from the size of the sample can not be ignored. A DUT must to some degree be large because of the manufacture of the DUT and the handling thereof. Further, the semi-rigid cables 110 and 111 must be connected to the two ends of the DUT 104 to obtain matching of the characteristic impedance of 50Ω for the signal source and the terminator. Therefore, even a small DUT is several cm or larger in size. Since the MSL structure is a distributed constant circuit model, a PIM is not generated at one concentrated point on the MSL, but is accumulated in a distributed manner. Therefore, the PIM level measured by the PIM measurement unit 106 varies, depending on the length Ls of the MSL.
Furthermore, in a case wherein a plurality of PIM generation sources are locally present on the MSL, PIM signals produced by these PIM generation sources interfere with each other. As a result of this interference, the PIM level observed could be changed in accordance with the MSL length. In addition, nonlinearity at the contact point of the connectors and the soldering point of the above described cables is also an obstacle to obtaining a precise PIM evaluation using the sample. Moreover, the processing of the DUT is troublesome, and in a conventional PIM measurement system that assumes the performance of impedance matching condition using the terminator, it is difficult for a precise PIM measurement, for a conductive material, etc., to be performed.
One objective of the present invention is to provide a method whereby the operational influence of a matching terminator is removed and the highly sensitive detection of a PIM is enabled by employing a DUT, for which there are extremely few limitations in size and shape, and a measurement system therefor. The current density for generating a PIM can be easily quantified, and property evaluations can be performed not only for a metallic material, but also for other materials, like an electric material such as a dielectric or a magnet, and a magnetic material. In addition, the PIM measurement method of the present invention can be applied for a defective detection method for electronic devices.
Non-patent document 1: “Application of nonlinearity measuring method using two frequencies for contacts”, Isao Minowa, et. al., IEICE Transactions on Electronics, Vol. J85-C No. 11, pp. 91-924, November, 1985 Non-patent document 2: “PIM generated in dielectric circuit board that employs microstrip line”, Nobuhiro Kuga, et. al., IEICE Transactions on Communications, Vol. J88-B No. 4, pp. 847-852, April 2005